Abstract: A flow injection method for the fluorescence determination of L cysteine in the medicine injection has been developed based on the fluorescence quenches of the complex system between a new fluorescence reagent:5-(4-chlorophenylazo)-8-benzenesulfonamidoquinoline (CPBSQ) and Cu(II) cysteine.The determination is carried out at λex/λem=327/368nm.The sample frequency of the method is 64h-1 with a linear range of 0.05-5.5μg·mL-1 and a detection limit of 0.01μg·mL-1.The relative standard deviation (RSD) for the determination of 11 samples of 4.0 mg·L-1 cysteine is 1.52%.A lot of metal ions,protein and amino acid without-SH group do not interfere the determination.The proposed method has been applied to the determination of cysteine in the medicine injetion with satisfactory results.